Polyethylene naphthalate fibers exhibit high tenacity, high modulus and excellent dimensional stability, and is now being applied widely to the field of industrial materials including a tire cord and a rubber reinforcing material for a driving belt and the like. In particular, they are strongly expected as a substitute of rayon fibers having been conventionally used, owing to the high modulus. This is because the rayon fibers have such a problem that they generate large load on production and suffers difficulties on processing, molding and use due to the large difference between the wet and dry properties thereof. However, rayon fibers have high dimensional stability and are easy to handle as rubber reinforcing fibers, but polyethylene naphthalate fibers contain molecules that are rigid and liable to align in the fiber axis, thereby facilitating provision of such properties as high tenacity and high modulus, but have such a problem that the dimensional stability, particularly the dimensional stability to heat, is difficult to attain simultaneously.
Under the circumstances, for example, Patent Document 1 proposes polyethylene naphthalate fibers that are excellent in heat resistance and dimensional stability formed by high-speed spinning. However, there is a problem that the fibers have low strength when they have a high melting point, but the fibers have a low melting point when they have high strength. In other words, the fibers cannot satisfy both strength and heat resistance at high levels.
Patent Document 2 discloses polyethylene naphthalate fibers that are excellent in hot air shrinkage and creep ratio along with high strength formed by providing a heated spinning chimney heated to 390° C. immediately beneath the melt-spinning die (spinneret) to perform high-speed spinning and hot stretching at a draft of about 300 times. However, the resulting fibers still have a low melting point of 288° C. and an insufficient tenacity of 8.0 g/de (about 6.8 N/dtex), and thus are not satisfactory in heat resistance and dimensional stability.
As different from Patent Document 2, Patent Document 3 proposes polyethylene naphthalate fibers that have high strength and excellent heat stability formed in such a manner that an undrawn yarn formed with a drawing speed of 1,000 m/min or less and a low draft of about 60 times is subjected to delayed cooling with a spinning chimney having a length of from 20 to 50 cm and an atmospheric temperature of from 275 to 350° C., and then to drawing at a high draw ratio. Patent Document 4 proposes polyethylene naphthalate fibers that have high strength and excellent dimensional stability formed in such a manner that an undrawn yarn having a low birefringence of from 0.005 to 0.025 is obtained at a spinning draft ratio of from 400 to 900, and is then subjected to multi-stage drawing at a total draw ratio of 6.5 or more.
However, fibers obtained by these methods have favorable properties in strength, but the melting point thereof is as low as 284° C. or lower, and thus they are still insufficient in heat resistance and dimensional stability.    (Patent Document 1) JP-A-62-156312    (Patent Document 2) JP-A-06-184815    (Patent Document 3) JP-A-04-352811    (Patent Document 4) JP-A-2002-339161